This invention relates generally to antennae and particularly relates to a vertical dipole antenna suitable for the ELF range.
The ELF (extremely low frequency) range extends below 1,000 Hz to approximately 10 Hz. It has long been recognized that this frequency range is particularly suitable for long range communications and for communications with a vessel under the surface of the ocean. In this connection reference is made to a paper by John Merrill entitled "Some Early Historical Aspects of Project Sanguine" which appears in IEEE Transactions on Communications, Volume COM-22, No. 4, April 1974, pages 359-363.
Radiating at the ELF range can be used not only for communication under the ocean, but also for geophysical exploration because the radiation penetrates through a greater depth. Project Sanguine was based on a horizontal antenna of great length and suitably grounded. Similar devices have been proposed in the patent to Gutton et al., U.S. Pat. No. 3,670,247. The antenna proposed here is intended to radiate from the surface of the water.
An ELF antenna for use on the ground has been proposed in the patent to Tanner U.S. Pat. No. 3,215,937. The antenna is loaded by space capacitors and fed by a center inductor. Also, a closed configuration has been proposed. However, again the antenna is a horizontal antenna.
It can be shown that a vertical dipole is considerably more efficient and permits to radiate greater power than a horizontal antenna. One of the reasons is that a vertical or electric dipole couples more readily with the waveguide formed by the ground and the ionosphere. Thus, the improvement of a vertical over a horizontal dipole is proportional to (.lambda./S).sup.2, where .lambda. is the wavelength and S the size of the radiator. More specifically the power radiated by a vertical electric dipole divided by the power radiated by a magnetic dipole is 10.sup.6 (l/L).sup.2. In this formula l is the length of the vertical dipole antenna and L is that of the end grounded horizontal antenna. It is assumed for this calculation that the wavelength is 3,000 Km (kilometers) and the skin depth of the ground is 2 Km. Hence, by way of example, a vertical antenna 1 kilometer long will radiate as much power as a horizontal antenna having a length of 1000 Km and carrying the same current. If the horizontal antenna is not grounded, the same conclusion holds true.
Various vertical antennas have been disclosed in the prior art. Thus, reference is made to the patent to Eisenstein U.S. Pat. No. 930,746. This antenna was intended for a frequency of 30 khz which is well above the ELF range. Both a coil and a capacitor are at the top of the antenna; however, the patent does not disclose any manner of modulating such an antenna.
Another early patent to Kitsee U.S. Pat. No. 651,361 shows a balloon for carrying the high voltage coils for an antenna. The inductor is disposed near the balloon. Modulation takes place by telegraphy or the like. However, the frequency range has not been indicated.
The patent to Kline U.S. Pat. No. 3,372,395 does relate to a VLF (30 khz) antenna which is above the ELF range. The antenna is of the inductive type and consists of an inductor wound about a magnetic core to provide a large inductance. The patent to Hafner U.S. Pat. No. 3,566,317 discloses a surface wave transmission line having a winding mechanism to vary its length.
It is also proposed to utilize an airplane carrying the line in a floating structure to move the antenna through the air. The patent to Schaad et al. U.S. Pat. No. 3,660,760 discloses among other things an inductive communication system consisting of horizontal wires and a switching device for the purpose of selectively adding a conductor to the system. This in turn will vary the inductive feed pattern.
It is accordingly an object of the present invention to provide a vertical dipole antenna suitable for use in the ELF range.
Another object of the present invention is to provide such an antenna which is more efficient than a horizontal antenna whether end grounded or not and which may be frequency or phase modulated.
A further object of the present invention is to provide a vertical dipole antenna for ELF communications which may consist of a plurality of synchronized radiators thereby to increase the radiated power substantially over that of a single antenna to form a super radiant antenna.